The maturation of spinal dorsal horn nociceptive neuronal network activity over the postnatal period.

The dorsal horn (DH) of the spinal cord is physiologically immature at birth. Spinal excitability increases and wide dynamic range (WDR) neurons in lamina V have lowered activation thresholds and larger receptive field sizes. The DH is composed of 5 laminae containing diverse interneuronal populations yet our understanding of the physiology of the DH is based on behavioural studies or extrapolation of single cell WDR recordings to the whole network. We have employed multi-electrode arrays to record contiguous activity across the whole DH in neonatal juvenile, adolescent, and young adult Sprague-Dawley rats after electrical stimulation of the ipsilateral hindpaw (0.01-5 mA). Baseline DH activity increased significantly (P < 0.0001) over early life, as did DH responses in the post-stimulus period corresponding to A-fibre-mediated input. Age-dependent differences in DH activity during this period were not significantly different in the superficial and intermediate DH, but were in the deep DH. C-fibre-mediated activity was more variable and, unlike the A-fibre period, there were no significant differences in stimulus response profiles. However, significant C-fibre-evoked activity was detected in juvenile, adolescent, and adult groups across the whole array and particularly in the deep DH (P < 0.05 and P < 0.01). Finally, short-term plasticity ("wind-up"), previously observed with single-unit approaches was detected in all regions of the adult rat DH, but not in younger ages. Despite behavioural and single-cell responses being more excitable in early life, at a network level the DH is significantly less excited than in adulthood.